The world number three went out in 32 to signal his intentions and two more birdies against one bogey on the back nine, as the bad weather came in, nudged him ahead atop the leaderboard at five-under 67.
It was the first time the 38-year-old Englishman had led after the first round of a Major despite having five top-three finishes in his last nine majors.
Westwood finished the day one shot ahead of South African Louis Oosthuizen, who had four birdies in the last five holes to card a 68, and Peter Hanson of Sweden.
A stroke further back were a group of players comprising Paul Lawrie of Scotland, Miguel Angel Jimenez of Spain, Francesco Molinari of Italy and Americans Ben Crane, Jason Dufner and Bubba Watson.
The 2008 winner, Zach Johnson of the United States, was on 70, level with veterans Vijay Singh and Jim Furyk among others.
Favorites Tiger Woods and Rory McIlroy both struggled.
Woods battled against a wayward driver to scramble a level par 72 while McIlroy opened with a double-bogey six and splashed into Rae's Creek in front of the 13th green en route to an edgy 71.
Westwood, runner-up here to Phil Mickelson two years ago, said that he had been working at easing the tension involved in playing in the Masters.
"I've taken it very relaxed this week. I've played here enough -- it's my 13th time -- and I know the golf course and that eradicates a lot of the tension," he said.
"I came close recently and I have had a good career and all there is to win is a major championship and that is my primary focus now."
Oosthuizen, the 2010 British Open champion, said he was surfing on the wave of recent South African successes at Augusta National, including close friend Charl Schwartzel's four-birdie finish to win last year.
"We play together a lot and I think it's more just watching videos of him winning over and over, and seeing the way he handled himself under the pressure which was brilliant, finishing with four birdies," he said of his fellow South African's influence on him.
Woods, meanwhile, was wayward off the tee and had his short game to thank for salvaging a 72.
"I hit some of the worst golf swings I've ever hit today, and that's all right," he said.
"I just hung in there and grinded my way around the golf course and stayed very patient, stayed in the moment.
"Unfortunately that was about as good as I got right there. I could have shot one, maybe two better, but I got a lot out of that round."
Woods, who is seeking his 15th major title to draw to within three of Jack Nicklaus's record of 18, won his first tournament in 28 months two weeks ago and was immediately installed as Masters favorite with McIlroy leading a strong European challenge.
McIlroy, who blew a four-stroke lead on the Sunday in last year's Masters, seldom looked comfortable after pushing his opening drive into the tall Georgia pines lining the first hole.
But after a mixed bag of a round, he finished strongly with back-to-back birdies to duck under par.
"I'm a lot more pleased coming off the golf course than if I had finished par and to come off under-par is pleasing," the 22-year-old US Open champion said.
"My perseverance and patience paid off and it was nice to finish as I did, but I'm surprised someone didn't go lower than five today."
World number one Luke Donald, meanwhile, survived a disqualification scare after an investigation ruled that an administrative error was to blame for the wrong posting of his score at 73 instead of 75.
But the Englishman's four-over total left him with a mountain to climb as he seeks to win his first Major.
"I’m disappointed. That’s not what I wanted. My irons were a bit off. Here that can eat you up. Maybe I can find something tomorrow," Donald said.
Three-time former champion Phil Mickelson was also left with a battle on his hands just to make the cut.
He stumbled badly with a triple bogey seven at the 10th, where he lost his ball off the tee, and with his driver deserting him, he came in with a two-over 74 that included a long birdie putt at the last.
First-round strugglers also included last year's joint runners-up Jason Day and Adam Scott of Australia, who carded 76 and 75 respectively, and 2008 champion Trevor Immelman of South Africa, who had a 78.